The process that, the equipment regulates internal voltage's amplitude and frequency (A/F) to generate changes in the electric network terminal power, plays a critical role in determining a power system's dynamic behavior. Traditional system analysis relies on the “quasi-steady state” assumption of constant frequency to investigate the network, and the results are often reasonable due to the network's time constant being fast enough compared to the internal voltage dynamics. However, the increasing integration of converter-interfaced equipment with fast A/F dynamics raises concerns about the network's power variations concerning the A/F, including the characteristics of potentially impacting power system dynamics. To address these concerns, this paper emphasizes the demands for analyzing the network's stimulation-response relationships under the dynamic practice of time-varying A/F. It provides a generalized method to establish explicit expressions for the original stimulation-response relationship of the network. On this basis, the dynamic characteristics potentially impacting the system dynamic processes are analyzed and elaborated. In addition, this paper identifies differences in the system's dynamic behaviors that may arise from commonly used static network relationships, even for system electromechanical dynamics. This work aims to lay the foundation for further understanding of the physics and analytics of the network under the equipment-regulated time-varying A/F.

The effect of frequency variation on system stability becomes crucial when a voltage source converter (VSC) is connected to a weak grid. However, previous studies lack enough mechanism cognitions of this effect, especially on the stability issues in DC voltage control (DVC) timescale (around 100 ms). Hence, this paper presented a thorough analysis of the effect mechanism of frequency variation on the weak grid-connected VSC system stability in a DVC timescale. Firstly, based on instantaneous power theory, a novel method in which the active/reactive powers are calculated with the time-varying frequency of voltage vectors was proposed. This method could intuitively reflect the effect of frequency variation on the active/reactive powers and could also help reduce the system order to a certain extent. Then, a small-signal model was established based on the motion equation concept, to depict the effect of frequency variation on the weak grid-connected VSC system dynamics. Furthermore, an analytical method was utilized to quantify the effect of frequency variation on the system’s small-signal stability. The quantitative analysis considered the interactions between the DC voltage control, the terminal voltage control, phase-locked loop, and the power network. Finally, case studies were conducted, and simulation results supported the analytical analyses.

With the increasing challenges posed by line-commutated converter-based high-voltage direct current (LCC-HVDC) in the era of widespread renewable energy utilization, AC/DC dynamic interaction problems under controller regulations appeared more often. However traditional approaches often fall short in identifying the features of signals involved in AC/DC conversion in grid dynamics, and the characteristics of the conversion process in the LCC main circuit are overlooked. Consequently, this paper aims to fill this gap to help reveal the mechanism of grid dynamic problems dominated by controller regulations. Based on the recognition that the amplitude/frequency of AC electrical variables has time-varying characteristics in practice during grid dynamics, this paper focuses on the conversion mechanism between AC and DC electrical variables and then the basic features of electrical variables in LCC main circuits during grid dynamics are expressed using time-varying amplitude/frequency rotating vectors. First, the six-pulse firing pattern is expressed using the rotating vector with time-varying frequency. Then the time-varying amplitude/frequency rotating vectors are utilized to discuss how AC/DC voltage and current converse, including the power produced from them during grid dynamics. Moreover, simulations are performed to validate the efficacy of the mechanism, alongside elucidating its potential applications to benefit grid dynamic analysis. Finally, the salient contributions of this proposed framework are clarified.

The dynamic behavior of large-scale systems containing diverse devices is a crucial focus for system operators, particularly in power systems where grid-connected devices generate AC electrical signals through various modulated methods. One of the main objectives of power systems is to transmit power. To this end, time-varying amplitude/frequency rotating vectors are used to describe different AC voltage and current signals, and then the active/reactive power separation mechanisms and characteristics for different signal-modulated power devices are explored. These mechanisms and characteristics are analyzed and verified through time-domain simulations. The theoretical contribution of this paper is that it thoroughly clarifies the misconception in current power theories by demonstrating that active power and reactive power naturally arise as inherent physical quantities rather than being solely mathematically defined. In terms of practicality, this paper can provide physically grounded insights for the power calculation methods and offer guidance for the design of power measurement in actual power system dynamic processes. Through the analysis presented in this study, the analysis, measurement, and control of the active/reactive components in renewable energy equipment based on the instantaneous reactive power calculation method or the traditional sinusoidal steady-state power calculation method do not need to be updated.

The increasing integration of power electronic converters in modern power systems has significantly accentuated stability concerns. Among these, the synchronization stability challenges of grid-following (GFL) converters under weak grid conditions are particularly prominent. It has been demonstrated that the introduction of grid-forming (GFM) converters can effectively mitigate these stability issues. However, a theoretical understanding of how various GFM control strategies impact GFL stability is still incomplete, making the small-signal stability assessment of hybrid GFL-GFM systems a significant challenge. Moreover, the reactive power control (RPc) of GFM converters plays a crucial role in maintaining system stability. Research on how RPC affects its support capability remains unexplored. To address these issues, this paper proposes a stability assessment method for GFL-GFM hybrid systems. By employing modal decoupling and matrix perturbation methods, a decoupled low-order system is developed for characterizing the stability of the original system, thereby reducing the analytical complexity and offering physical insights into the supporting capabilities of GFM converters. Furthermore, an enhanced RPC structure for GFM converters is proposed to improve their grid-support capability within different frequency ranges. The effectiveness of the proposed method is validated on the IEEE 39-Node system.